EVOLUTION OF DEEPLY SUBDUCTED CARBONATES AND THE FORMATION OF MICRODIAMOND

UHPM carbonate rocks from the Kokchetav Massif, northern Kazakhstan is the best sample to understand the evolution of deeply subducted carbonate sediments due to continental collision and the carbon recycling and CO2 behavior from Earthfs surface to the mantle. Those rocks demonstrated that carbonates could be subducted into the depth over 200km and that diamond could form in the carbonate systems with metamorphic fluids under UHP conditions. The following three types of carbonates have been described from the Kokchetav Massif. 1) Dolomite marble: many of the samples contain abundant microdiamond and some lack diamond. Diopside (+ K-feldspar/phengite/phlogopite lamellae) + dolomite assemblage is stable. 2) Dolomitic marble: aragonite + Ti-clinohumite tie-line is stable and no diamond occurs. Diopside lacks K2O-bearing silicate lamella. 3) Calcite marble: this rock lacks diamond and dolomite. Titanite contains exsolved needles and plates of coesite, and this indicated the supersilicic composition of precursor titanite at P > 6GPa. Diopside contains exsolved K-feldspar and phengite. The precursor diopside could contain H2O (>1000ppm). The stabilities of Ti-bearing phases are distinct in each carbonates: rutile in dolomite marble, Ti-clinohumite in dolomitic marble, and titanite in calcite marble. The titanite and rutile stabilities were controlled by a solid-solid reaction: dolomite + titanite (low-P side)=aragonite + diopside + rutile (high-P side). Aragonite and Ti-clinohumite assemblage indicates extremely low XCO2 conditions in contrast to the assemblage diopside + dolomite. A continuous sample that has different assemblages part by part clearly indicated the heterogeneity of fluid condition in mm to cm scale, and this might be caused by H2O-rich fluid infiltration during UHP metamorphism. The microdiamond formed at two stages and the 2nd stage formation might be closely related to UHPM fluid. The model of fluid evolution from CO2-rich to H2O-rich compositions during prograde stage can explain the metamorphic history of the Kokchetav carbonate rocks. Coesite inclusion in zircon in diamond-bearing dolomite marble is evidence for the low degree of dilution of CO2-rich fluid at prograde UHP conditions. Such UHPM fluid evolution may be caused by the dehydration in gneisses surrounding the carbonate.